1. Field of the Invention
This invention relates to a method of forming patterns.
2. Description of the Related Art
In the field of electronic parts which need various fine workings, such as semiconductor integrated circuits, fine working technology including photolithography is employed. The technology is carried out, for example, in accordance with the following process.
First, a resist layer is formed on a semiconductor substrate, such as siliconwafer and the like, by a spin-coating method.
Next, the resist film is patternwise exposed to cause a photochemical reaction thereon, developed and rinsed to form a desired resist pattern.
Further, using the resist pattern as an etching resistant mask, the exposed area of the substrate is etched to form fine lines and spaces (i.e. open area).
Recently, in order to attain large-scale integration with elevation of the density of electronic parts, it has been required to form fine resist patterns of submicron orders. As one means of satisfying the request, a light source (ray source) of shortened wave length has been employed. For instance, a process of forming fine resist patterns by using an ArF excimer laser (having a wavelength of 193 nm) or fifth harmonic wave of YAG laser (having a wavelength of 213 nm) has been developed.
In the above-mentioned process, it is requested to form resist patterns having an excellent dry-etching resistance for the purpose of more effectively carrying out fine working of the desired patterns. Regarding the point, it is known that use of a light-sensitive composition containing a compound having aromatic ring(s) or an aromatic compound as a resist material is effective. As such a resist material, for example, there are mentioned a resist composition containing an alkali-soluble novolak resin and a light sensitive agent of quinone diazide, which is used in the current mass-production process of producing LSI, and a chemically amplifiable resist composition containing an alkali-soluble resin having phenolic nucleus/nuclei, a dissolution inhibitor and an optical acid-generating agent, which is disclosed in Japanese Patent disclosure No. 63-27829.
In conventional methods of forming patterns, exposure with the above-mentioned short-wave ultraviolet rays and the like, using a light-sensitive composition containing an aromatic compound as a resist material, for forming resist patterns having a high resolving power and an excellent dry-etching resistance has been attempted.
However, the above-mentioned resist material or a light-sensitive composition containing an aromatic compound especially has an extremely strong absorption of light within a short wavelength range (about 190 nm or so) since the electrons of the aromatic ring in the compound are excited with the light. Therefore, in the process of forming patterns where the resist material (resist film) is exposed with a light source having the above-mentioned short-wave length lights, the material often has over-absorption of the light during exposure. Therefore, the photochemical reaction could not advance sufficiently through the resist film.
This phenomenon will be explained in more detail hereunder. It is known that the aromatic compound contained in the above-mentioned resist material generally has three absorption bands to be caused by the structure of the aromatic ring of the compound. Of them, the first and second absorption bands from the long-wave side are caused by the electronic excitation as generated by molecular vibration, which should not be generated from the intrinsic molecular symmetry, and therefore they are not so strong. On the other hand, the third absorption band is extremely strong. In the conventional methods of forming patterns, since the exposure is effected with ultraviolet rays or the like having a wavelength range near to the first and second absorption bands from the long-wave side or having a wavelength range further longer than these absorption bands, the above-mentioned resist film does not have any strong light absorption. However, where exposure with short-wave radiation rays or the like is employed, the above-mentioned resist film would probably have a strong light absorption due to the third absorption band of the above-mentioned aromatic compound. In particular, where the above-mentioned resist film is exposed with an ArF excimer laser the like, the light transmittance through the film is extremely small so that the intended photochemical reaction could not advance throughout the whole film. As a result, a resist pattern having a desired resolving power and also having an excellent dry-etching resistance could not be obtained, which is a problem.
The present invention has been made in view of the above-mentioned problems.